1. Field of the Invention
The present invention relates generally to the art of castable resins which contain reactive sites capable of urethane chain extension or crosslinking and which also contain unsaturation, preferably in the polyester component of the resin, to provide improvement to the final resin physical properties through the mechanism of free radical crosslinking of the unsaturated sites. More specifically, the invention relates to hybrid castable resins which include terephthalic moities in the polyester chain, and still more specifically to such resins prepared using secondary hydroxyl containing materials for preparation of the polyester component.
2. Description of the Prior Art
Hybrid polyester resins have been known for several years and are described in considerable detail by H. R. Edwards in papers presented in 1984 and 1987. See "Handling and Physical Properties of Hybrid Polyesters" reported at Session 8-C, Pages 1-8 of the proceedings of the 39th Annual Conference/Reinforced Plastics/Composites Institute, The Society of the Plastics Industry, Inc., Jan. 16-19, 1984 and "The Application of Isophthalic Unsaturated Polyester Urethane Hybrids in Conventional Molding Techniques"--Session 8-C, Pages 1-6, 42nd Annual Conference/Composites Institute, The Society of the Plastics Industry, Inc., Feb. 2-6, 1987.
In his papers, Edwards points out the versatility of hybrids which result from the reaction of unsaturated polyester glycols with diisocyanates diluted in styrene monomers. He explains that early work on the concept of blending urethane and unsaturated polyester technologies did not immediately result in commercial products. Amoco Chemicals Company, Edward's employer, introduced isophthalic unsaturated polyester urethane hybrids to overcome the commercial viability problem and was still producing commercial systems of this type up to the time of this invention.
The Amoco systems comprise three low molcular weight unsaturated polyesters with available hydroxyl groups, which are diluted with styrene monomer. The glycols used are ethylene glycol, diethylene glycol, and neopentyl glycol. These polyesters are reacted with polyisocyanates such as a modified MDI or a polymeric MDI (4'4-diphenylmethane diisocyanate). Edwards discusses glycol and acid selection in terms of equivalent weight and geometry and the use of the hybrid materials in machine casting, resin transfer molding, reaction injection molding, spray-up, pultrusion and press molding. He concludes that work through the date of his last paper has resulted in commercialization of a resin having high impact strength and high modulus, along with ease of handling. He also recognizes that "even at lower equivalent weight, the use of--glycols containing secondary hydroxyls results in embrittled cured hybrids" (1987 paper, page 1) and focuses attention on the use of isophthalic acid to provide "the best compromise of styrene compatability with the proper configuration for linear molecular weight development" (1987 paper, page 1).
The physical properties of the resins described by Edwards are quite good, but his systems suffer from several drawbacks when compared to the systems suggested by the present invention. Foremost among the disadvantages of the earlier systems is a relatively low heat distortion temperature (HDT) for resins with notched Izod impacts of 1.0 or greater. In addition, the present invention provides for the use of relatively inexpensive starting materials for the polyester, i.e. the use of terephthalic acid groups, and the ability to use, and even the desirability of using, secondary hydroxy containing materials for polyester formation. The present invention departs from the teachings of the Edwards work and yields lower cost hybrid castable polymer systems having even better properties. The resins of the present invention, possessing very desirable "toughness" and substantially improved heat distortion temperatures, represent a substantial breakthrough in the art.
It would also be desirable to use materials other than styrene as the unsaturated diluent, and the present invention contemplates the use of vinyl toluene for such purposes.